The goal of the proposed experiments is to understand the organization of the luteinizing hormone-releasing hormone (LHRH) neurosecretory system, which constitutes a neural pulse generator controlling the episodic secretion of luteinizing hormone (LH) by the anterior pituitary. In the sheep, the LH-pulse generator is the central mechanism by which photoperiodic and hormonal signals regulate reproduction. We hypothesize that plasticity in the synaptic arrangements of LHRH neurons might underlie changes in the physiology of the LH-pulse generator responsible for seasonal breeding in this species and others. Light microscopic observations have revealed that LHRH neurons in the sheep have a complex morphology unlike those cells in any other species studied to date. Comparisons between intact anestrous ewes and breeding season ewes, during the mid-luteal phase of their estrous cycle, have suggested the possibility of seasonal alterations in the morphology of LHRH neurons. We propose to extend these observations to the electron microscopic level, by employing ultrastructural immunocytochemistry (ICC) to quantify the number and type of synapses contacting LHRH cells and dendrites, and by comparing these measures between intact anestrous and mid-luteal phase ewes. We will combine ultrastructural ICC with radiolabeling techniques, in order to determine the neurochemical identity of synapses which directly contact LHRH cells and dendrites. We will also examine LHRH cells for direct evidence of close appositions between adjacent LHRH cells or dendrites, which might serve as a morphological basis for their coordinated pulsatile activity. Finally, if we observe seasonal differences in the synaptic arrangements of LHRH cells between intact anestrous and mid-luteal ewes, we will determine whether these changes are due to the influence of photoperiod alone (steroid-independent effect), or to the long-term presence of estradiol and/or progesterone (steroid-dependent effect).